Is there a way to Add-On features ala carte?

I am a consultant with no employees (team). So an Enterprise subscription is just too big and expensive for me. Is there any way that I can "Add-On" Custom Domains with any other plan?

We don't have add-ons on our plans. The regular plans has the same features and only differs in their own set of limits. And unfortunately, custom domain feature is only available on Enterprise plans.

As a workaround, you can embed your forms into your own webpages so you can just share your website URL instead of the forms'.

Here's a guide on embedding-a-form-to-a-web-page.